The present invention is related to silicone compositions. More particularly, the present invention is related to low viscosity, curable polydiorganosiloxane compositions.
Dispensible materials that can cure and give high thermal conductivity are typically used in the electronics industry. Currently, there are two classes of cured articles used as thermally conductive sinks. Sakamoto et al., Japanese Patent No. 05117598, discuss highly filled matrices that are cured to make a pad. The pad can be cut and physically placed in an electronics device. Toya, Japanese Patent No. 02097559, discuss a filled matrix that is dispensed and cured in place. The dispensable approach requires that the material have a viscosity that is low enough such that the material can be forced through an orifice for rapid manufacture of many parts. However, the final cured product must have sufficiently high thermal conductivity.
There remains a need to find a material that has a sufficiently low viscosity such that it can be rapidly placed on a small device with high power requirements. The high power requirement needs a way to remove more heat. This requirement necessitates a thermally conductive material. Thus, dispensable, curable, and high thermally conductive materials are constantly being sought.
The present invention provides a polydiorganosiloxane comprising the general formula:
(R4)2R5SiO[(R4)2SiO]m[R4R5SiO]nSi(R4)2R5 
wherein R5 is a polar radical with a dipole moment great than about 2 debye; R4 comprises C1-8 alkyl radicals, phenyl radicals, vinyl radicals or mixtures thereof; and xe2x80x9cmxe2x80x9d+xe2x80x9cnxe2x80x9d has a value sufficient to provide a polydiorganosiloxane composition with an initial viscosity in a range between about 100 centipoise and about 50,000 centipoise at 25xc2x0 C.
The present invention further provides a silicone composition comprising a curable adhesive formulation which comprises
(A) a polydiorganosiloxane comprising the general formula:
(R4)2R5SiO[(R4)2SiO]m[R4R5SiO]nSi(R4)2R5 
wherein R5 a polar radical with a dipole moment greater than about 2 debye; R4 comprises C1-8 alkyl radicals, phenyl radicals, vinyl radicals or mixtures thereof; and xe2x80x9cmxe2x80x9d+xe2x80x9cnxe2x80x9d has a value sufficient to provide a polydiorganosiloxane with an initial viscosity in a range between about 100 centipoise and about 50,000 centipoise at 25xc2x0 C.;
(B) at least one thermally conductive filler; and
(C) at least one diluant
wherein the total silicone composition has a viscosity in a range between about 10,000 centipoise and about 250,000 centipoise at 25xc2x0 C.
In yet a further embodiment of the present invention, there is provided a method for substantially increasing the thermal conductivity of a silicone composition comprising:
providing at least one polydiorganosiloxane wherein the polydiorganosiloxane has the general formula:
(R4)2R5SiO[(R4)2SiO]m[R4R5SiO]nSi(R4)2R5 
wherein R5 a polar radical with a dipole moment great than about 2 debye; R4 comprises C1-8 alkyl radicals, phenyl radicals, vinyl radicals or mixtures thereof; and xe2x80x9cmxe2x80x9d+xe2x80x9cnxe2x80x9d has a value sufficient to provide a polydiorganosiloxane with an initial viscosity in a range between about 100 centipoise and about 50,000 centipoise at 25xc2x0 C.; and combining into the polydiorganosiloxane at least one thermally conductive filler in a range between about 60% by weight and about 95% by weight of the total composition wherein the total silicone composition has a viscosity in a range between about 10,000 centipoise and about 250,000 centipoise at 25xc2x0 C.
In yet a further embodiment of the present invention, there is provided a thermal interface material comprising:
(A) at least one polydiorganosiloxane comprising
(R4)2R5SiO[(R4)2SiO]m[R4R5SiO]nSi(R4)2R5 
wherein R5 a polar radical with a dipole moment greater than about 2 debye; R4 comprises C1-8 alkyl radicals, phenyl radicals, vinyl radicals or mixtures thereof; and xe2x80x9cmxe2x80x9d+xe2x80x9cnxe2x80x9d has a value sufficient to provide a polydiorganosiloxane with an initial viscosity in a range between about 100 centipoise and about 50,000 centipoise at 25xc2x0 C.;
(B) at least one thermally conductive filler; and
(C) at least one diluant
wherein the thermal interface material provides adhesion to at least one substrate.
It has been found that the use of a polydiorganosiloxane with polar-functional groups provides a substantially lower initial viscosity. xe2x80x9cSubstantially lower initial viscosityxe2x80x9d as used herein refers to a polydiorganosiloxane with a viscosity in a range between about 100 centipoise (cps) and 50,000 centipoise at 25xc2x0 C. When the polydiorganosiloxane of the present invention is combined with a diluant, at least one thermally conductive filler, and optionally, a cure catalyst, a total silicone composition is formed with a substantially higher thermal conductivity.
xe2x80x9cSubstantially higher thermal conductivityxe2x80x9d as used herein refers to a composition with a thermal conductivity greater than about 1.5 Watts per meter per degree Kelvin (W/mK). The xe2x80x9cviscosity of the total silicone compositionxe2x80x9d as used herein typically refers to a viscosity before cure of the composition in a range between about 10,000 centipoise and 250,000 centipoise and preferably, in a range between about 20,000 centipoise and about 100,000 centipoise at 25xc2x0 C.
The polar-containing polydiorganosiloxane has the general formula (I),
(R4)2R5SiO[(R4)2SiO]m[R4R5SiO]nSi(R4)2R5xe2x80x83xe2x80x83(I) 
wherein R5 is a polar radical with a dipole moment greater than about 2 debye; R4 comprises C1-8 alkyl radicals, phenyl radicals, vinyl radicals, or mixtures thereof; and xe2x80x9cmxe2x80x9d+xe2x80x9cnxe2x80x9d has a value sufficient to provide an initial viscosity in a range between about 100 centipoise and about 50,000 centipoise at 25xc2x0 C. and a polar content in a range between about 1% by weight and about 10% by weight of the polar-containing polydiorganosiloxane. Radicals represented by R4 are preferably C1-4 alkyl radicals and more preferably, methyl. Typically, the polar-containing polydiorganosiloxane is present in a range between about 0.5% by weight and about 5% by weight of the total composition, and more typically in a range between about 1% by weight and about 2% by weight of the total composition.
The polar radical is typically a cyano functional group, an epoxy functional group such as cyclohexyloxy and 7-oxabicyclo[4,1,0]hept-3-yl, and glycidoxy, an acryloxy functional group, a methacryloxy functional group, a urethane group or combinations thereof. Most typically, the polar radical is an acryloxy, methacryloxy, or epoxy functional group.
Additionally, a reactive organic diluant may be added to the silicone composition to decrease the viscosity of the composition. Examples of diluants include, but are not limited to, styrene monomers such as tert-butyl styrene (t-Bu-styrene), (meth)acrylate monomers such as methylmethacrylate and hexanedioldiacrylate, methacryloxy-containing monomers such as methacryloxypropyltrimethoxysilane, epoxy-containing monomers such as biscyclohexaneoxypropyldimethylsiloxane, and glycioxy-containing monomers such as glycidoxypropyltrimethoxysilane. It is to be understood that (meth)acrylate includes both acrylates and methacrylates. The mixture of the diluant and the polar-containing polydiorganosiloxane lowers the viscosity, which allows for higher loading of filler. The amount of filler in the silicone composition is directly proportional to the thermal conductivity. Thus, the higher the amount of filler in the silicone composition, the greater the thermal conductivity of the silicone composition.
The thermally conductive fillers in the present invention include all common thermally conductive solids. Examples of thermally conductive filler include, but are not limited to, aluminum oxide, aluminum nitride, boron nitride, diamond, magnesium oxide, zinc oxide, zirconium oxide, silver, gold, copper, and combinations thereof. Typically, the thermally conductive filler is aluminum oxide, aluminum nitride, boron nitride, or diamond. The filler is present in a range between about 60% by weight and about 95% by weight of the total composition, more typically the filler is present is in a range between about 75% by weight and about 85% by weight of the total composition.
Cure catalysts may also be present in the total silicone composition that accelerates curing, of the total silicone composition. Typically, the catalyst is present in a range between about 10 parts per million (ppm) and about 2% by weight of the total composition. Examples of cure catalysts include, but are not limited to, peroxide, iodonium salts, and platinum catalysts. Curing typically occurs at a temperature in a range between about 50xc2x0 C. and about 175xc2x0 C., more typically in a range between about 100xc2x0 C. and about 150xc2x0 C., at a pressure in a range between about 1 atmosphere (atm) and about 5 tons pressure, more typically in a range between about 1 atmosphere and about 100 pounds per square inch (psi). In addition, curing may typically occur over a period in a range between about 5 minutes and about 1 hour, and more typically in a range between about 15 minutes and about 45 minutes.
The composition of the present invention may by hand mixed but also can be mixed by standard mixing equipment such as dough mixers, chain can mixers, planetary mixers, and the like.
The reaction of the present invention can be performed in batch, continuous, or semi-continuous mode. With a batch mode reaction, for instance, all of the reactant components are combined and reacted until most of the reactants are consumed. In order to proceed, the reaction has to be stopped and additional reactant added. With continuous conditions, the reaction does not have to be stopped in order to add more reactants.
Thermally conductive materials as described in the present invention are dispensable and have utility in devices in electronics such as computers or in any device that generates heat and where the device requires the heat to be efficiently removed. The thermally conductive material is typically used as a thermal interface material that provides adhesion to at least one substrate such as silicon, gallium arsenide (GaAs), copper, nickel, and the like.
In order that those skilled in the art will be better able to practice the invention, the following examples are given by way of illustration and not by way of limitation.